This invention relates to a compact fluorescent lamp with a discharge tube. The invention also relates to a method and apparatus for manufacturing the discharge tube of the lamp. The invention also concerns a molding tool used in the method and the apparatus.
Low pressure discharge lamps are well known in the art. These lamps contain small doses of mercury which radiates under the influence of the discharge arc. In order to accomplish maximum light output, it is desirable to make the discharge tube of the lamp as long as possible which results in a large luminous surface. At the same time, to accomplish light outputs comparable to the light output of a traditional incandescent bulb requires the application of discharge tubes with a length in excess of tens of centimeters. Such tubes must be folded into various forms in order to make them compact enough for various lamp shades and covers which were originally designed for incandescent bulbs.
It is known to divide a long discharge tube into shorter sections, and to dispose the sections in a parallel configuration around a common central axis. As an example, a 60 cm long discharge tube may be divided into six sections, each ten centimeter long. The tubes are connected to each other through connecting necks, alternating at the top end or bottom end of the tubes. This configuration has the advantage of utilizing a relatively large surface area of the tube sections because the adjacent tubes leave approximately two thirds of the tube surface visible from the outside of the lamp. This part of the tube surface may be considered as the effective tube surface. However, this parallel configuration is still too long for many applications because the housing of the lamp, which normally contains the ballast electronics, also adds to the overall length of the discharge lamp. Further, the discharge arc exerts non-uniform load on the discharge tube which adversely effects the light output and the lifetime of the lamp.
A double helix-shaped compact fluorescent lamp is disclosed in U.S. Pat. No. 5,705,883. Such a double helix configuration results in a much more compact discharge tube, and the overall length of a coil with a total length of approximately 60 cm may be reduced to six centimeters, instead of the ten centimeters necessary for the parallel configuration. However, the effective lighting area of the discharge tube is reduced, because approximately one half of the discharge tube surface is visible only to the outside apart from the top turn of the helix. Therefore, the luminous efficacy of this known lamp is not completely satisfactory.
In the lamp shown in U.S. Pat. No. 5,705,883, the double helix formed by the discharge tube has an essentially constant inner and outer diameter, apart from the sealed ends of the discharge tube which are bent inwards. The bending of the tube ends is made in a separate manufacturing step. The bending of the ends does not influence the effective surface of the discharge tube, as the bent ends are inserted into a supporting housing.
German Patent Application No. DE 41 33 077 discloses another double helix shaped discharge lamp. In this known discharge lamp, there is a cold chamber positioned at the top of the lamp between the two ends of the tube portions constituting the strands of the double spiral. There is a relatively large distance between the turns of the two strands of the helix, so that the surface of the discharge tube turning inwards is also partly visible, adding to the effective area. However, the large distance between the turns again results in a large longitudinal dimension of the coil to the detriment of the overall compactness of the lamp.
Another double helix shaped compact fluorescent lamp is disclosed in the document WO 94/29895. The double helix of this lamp has a central section and an end section. The sealed ends of the discharge tube are disposed at the end section. The inner diameter of the central section of the helix is smaller than the inner diameter of the end section. As a result, the overall lighting efficiency of the discharge tube does not surpass that of the discharge tube known from U.S. Pat. No. 5,705,883, while achieving shorter total discharge tube length due to the gradually decreasing diameter of the coil, and as a result, decreased light output. Generally, the lighting efficiency of this lamp is riddled with the same problems as the above mentioned lamps known from U.S. Pat. No. 5,705,883 and from German Patent Application No. DE 41 33 077. The compactness of the lamp is also less satisfactory, because the sealed ends add to the total length of the coiled discharge tube.
Therefore, there is a need for a discharge lamp which exhibits improved lighting efficiency combined with compact dimensions of the coiled discharge tube, particularly a short overall length of the coil formed by the discharge tube.
In an exemplary embodiment of the present invention, there is provided a compact fluorescent lamp comprising a double helix shaped discharge tube. The discharge tube includes two helix shaped tube portions which form the two strands of the double helix. The tube portions define a central axis of the discharge tube. The double helix constituted by the tube portions has a central section and a first end section. These sections of the helix are defined along the central axis, i. e. the sections are disposed along the central axis. The lamp base for receiving ends of the tube portions is disposed at the first end section. An inner diameter of the central section of the helix is larger than an inner diameter of the first end section.
The lamp with the discharge tube of the above-described design has an improved luminous efficacy combined with compact size and an aesthetic appearance, as compared with known discharge lamp designs. The two tube portions of the discharge tube forming the strands of the helix may be easily formed starting from a single integral glass tube, thereby avoiding imperfect joints between discharge tube sections.
In an exemplary embodiment of another aspect of the present invention, there is provided a method for manufacturing a coil shaped discharge tube for a compact fluorescent lamp. The method is applicable for the manufacture of discharge tubes in which the coil formed by the discharge tube has a larger inner diameter at a central section than an inner diameter at a first end section. The method comprises the following steps. A segmented molding core provided, and the segments of the core define an external envelope surface corresponding to the desired inner diameter of the coil. At least a part of the discharge tube is heated to a softening temperature, and the heated discharge tube is wound on the molding core. Thereafter, the discharge tube is cooled to a solidification temperature.
After cooling of the discharge tube, the segments of the molding core are withdrawn in a radial direction towards the center of the coil. Thereafter, the segments are withdrawn from the coil in an axial direction.
The proposed method is mostly useful when the coil is closed at an end towards the section with a larger diameter. This is the case with the proposed discharge tube which is formed as a double helix, and the sealed ends of the discharge tube are disposed at the section having a smaller inner diameter than a diameter of its central section.
In an exemplary embodiment of still another aspect of the present invention, there is provided an apparatus for manufacturing a coil shaped discharge tube for a compact fluorescent lamp, particularly the suggested double helix shaped compact fluorescent lamp. The apparatus is suitable for the manufacturing of discharge tubes in which the coil formed by the discharge tube has a larger diameter at a central section than a diameter at an end section. The apparatus comprises a molding core with an envelope surface corresponding to the inner surface of the discharge tube forming the coil. This molding core comprises segments which define the envelope surface of the core. The segments are arranged for displacement in a radial direction relative to the central axis of the coil.
The apparatus further comprises means for controlling the displacement of the segments in a radial direction relative to the central axis of the coil, and means for heating at least a part of the discharge tube to a softening temperature. The apparatus also has means for holding the discharge tube in an oriented position relative to the molding core and means for winding the softened discharge tube onto the molding core.
In an exemplary embodiment of still another aspect of the present invention, there is provided a molding core for manufacturing a coil shaped discharge tube, for use in the apparatus.
The method, the apparatus implementing the method and the molding core used in the apparatus ensures a fast and industrial-scale manufacturing of coil shaped discharge tubes, particularly double helix shaped discharge tubes in which the inner diameter of the double helix is smaller at the end of the helix than the inner diameter of a central section of the helix.